The present invention relates to a disk apparatus using a partial response maximum likelihood method for performing a waveform equalization so as to suppress an interference between bits of a read waveform by using a transversal filter known as an adaptive type equalizer and, more particularly, to a disk apparatus which can automatically adjust a waveform equalization amount of an adaptive type equalizer by using a training pattern which has been preformatted onto a disk medium.
Hitherto, in case of using a partial response maximum likelihood detecting method as a level detecting method, for example, a PR4ML (Partial Response Class 4 Maximum Likelihood) detecting method for a demodulating circuit of a magnetic disk apparatus, an optical disk apparatus, or the like, it is very important to accurately execute a waveform equalization including a transmission path. When the waveform equalization is not accurately executed, sampling points of a read waveform are deviated, so that an equalization error by the equalization occurs and an error rate is deteriorated by an amount corresponding to such an error. In the disk apparatus, electromagnetic converting characteristics of the transmission path including a disk medium and heads momentarily change due to a temperature change, an aging change, or the like. In order to compensate for such changes, an adaptive type waveform equalizer known as a transversal filter is used. At the head of a data frame of a disk medium, a training area in which a waveform pattern for determining a waveform equalization amount of the transversal filter by a training has been recorded is provided. A predetermined bit pattern for the detection of interference between bits is recorded in the training area. The transversal filter reads out a waveform of the bit pattern in the training area prior to the reading of a data frame, feed-back controls so as to minimize the equalization error, and adjusts tap coefficients. By such a training, the waveform equalization amount can be adjusted to the optimum equalization amount, adaptive to the electromagnetic converting characteristics of the transmission path, including the disk medium and the head at that time. After that, the equalization amount is fixed to the optimum state until the next training area is. The equalization amount is adjusted each time, as mentioned above, just before the data is read, thereby accurately executing the waveform equalization.
FIG. 1 shows a data demodulating circuit of a conventional magnetic disk apparatus using the partial response maximum likelihood detecting method. A read signal of a head 115 is amplified by an AGC amplifier 170. After that, the signal is filtered by a (1+D) filter 172 in an analog manner, and is sampled as digital waveform data by an A/D converter 174. The sampled data is subjected to a waveform equalization to suppress an interference between bits by an adaptive equalizer 176 having a transversal filter. Correct bit 0 or 1 is detected according to a maximum likelihood algorithm by a Viterbi demodulator 178. An error detector 180 executes a training operation for adjusting tap coefficients of a transversal filter so as to minimize an equalization error of the adaptive equalizer 176 on the basis of a read signal of a training pattern of the head 115. When a start instruction signal E12 which is generated when the start of the training pattern is detected by a "00" pattern detector 182 is received, the error detector 180 starts the training operation. A stop instruction signal E13 is generated when a pattern of a sync byte subsequent to the training pattern is detected by a sync byte detector 184. When a stop instruction E13 is received, the error detector 180 stops the training operation. The "00" pattern detector 182 and sync byte detector 184 enter an enable state by a read gate signal E11. As a training pattern, a method of repeating a "100110011" pattern a plurality of times is generally used, since an interference amount between bits can be easily detected.
FIG. 2A shows a recording pattern in the training area together with recording patterns before and after such a recording pattern. A training area 188 is provided after a preamble 186. In the training area 188, the "100110011" pattern is repeated twice. Subsequent to the training area 188, a pattern "100111111" of a sync byte 190 is provided and an ID portion 192 follows. The sync byte 190 is written as a flag to read ID information in the subsequent ID portion 192. In case of a reading operation, the read gate signal E11 of FIG. 2B is set to the H level at time t1 when a data frame starts and enters the enable state. The "00" pattern detector 182 outputs the detection signal E12, which is set to the H level at time t3, after the elapse of a pattern detection period T1 since the input of a "00" pattern at time t2 as shown in FIG. 2C, thereby starting the training operation by the error detector 180. Although a read signal of the pattern in the training area 188 in FIG. 2A is finished at time t4, the sync byte detector 184 which received a read signal of the next sync byte 190 detects a sync byte at time t6 after the elapse of a sync byte detection time T12 from time t5 when the sync byte 190 is finished and the detection signal E13 of FIG. 2D is set to the H level, thereby finishing the training operation by the error detector 180. That is, the training operation of the adaptive equalizer 176 starts at the first "00" pattern included in the training pattern and ends by detecting the next sync byte.
In such a training operation in the conventional magnetic disk apparatus, although the detection of the first "00" pattern in the training pattern is completed in a time of a few bits, a time of more than about 10 bits is necessary because the sync byte only detected after all of the sync byte patterns were read. On the other hand, the training operation of the adaptive equalizer 176 needs to be executed within the training pattern as much as possible. When the training operation is executed out of the training pattern, the error detector 180 erroneously operates so as to enhance the equalization error. Hitherto, although there is no problem in the start of the training from time t3 based on the "00" pattern detection signal E12 of FIG. 2C as mentioned above, since end time t6 by the sync byte detection signal El3 of FIG. 2D is deviated out of the training area 188, an erroneous training period T14 from time t4 to time t6 occurs. Therefore, by the unnecessary training operation of the error detector 180 in the erroneous training period T14, the adaptive equalizer 176 is adjusted to a state in which the equalization error is large, so that there is a fear such that the error rate cannot be guaranteed.
In a magnetic disk apparatus using an embedded servo system (data surface servo system) for detecting a head position by recording servo information on a data surface, the training pattern which is used for the equalization amount adjustment of the adaptive equalizer is ordinarily provided at the head of a data frame (sector). By providing the training area for every data frame, however, an area to store data is decreased by an amount corresponding to such training areas, so that a formatting efficiency deteriorates and an increase in memory capacity is disturbed.